docs: def to_strings(self):
          """
          Convert the expansion node to a list of expansion strings.
  
          :return: a list of expansion strings that represent the leaf nodes of the expansion tree.
          :rtype: list[union[str, unicode]]
          """
          result = []
  
          if not self.expansions:
              result.append(self.name)
          else:
              for expansion in self.expansions:
                  result.extend('{}.{}'.format(self.name, es) for es in expansion.to_strings())
  
          return result